216. Combination Sum III

Input: k = 3, n = 7
Output: [[1,2,4]]

combination和combination sum的结合题，结果需在0-9之间

public List<List<Integer>> combinationSum3(int k, int n) {
    List<List<Integer>> res = new ArrayList<>();
    List<Integer> path = new ArrayList<>();
    dfs(k, n, path, res, 1);
    return res;
}

public void dfs(int k, int n, List<Integer> path, List<List<Integer>> res, int pointer) {
    if (k == 0 && n == 0) {
        res.add(new ArrayList<>(path));
    } else {
        for (int i = pointer; i < 10; i++) {
            path.add(i);
            dfs(k - 1, n - i, path, res, i + 1);   
            path.remove(path.size() - 1);
        }
    }
}